A voltmeter is used to measure voltage of electronic components in the circuit.
Generally, while measuring a voltage on both ends of an electronic component, the voltmeter is connected parallelly on both ends of the electronic component. Because, the voltmeter has a particularly large resistance or is infinite. Thus, a current through the voltmeter is close to zero, and the voltmeter do not split an original current through the electronic component.
However, in fact, the resistance of the voltmeter is not infinite, and the voltmeter splits the current through the electronic component. Thus, while measuring a voltage on both ends of the electronic component by the voltmeter, an actual current value of the electronic component is less than a theory current value of the electronic component. So measuring the actual voltage value of the electronic component is also less than the theory current value of the electronic component.